The alkaline silver oxide-zinc secondary battery is of very great interest because of its potentially high watt-hour per pound output. However, the practical application of this battery has been limited by its short cycle life. This drawback is generally ascribed to separator failure. Specifically, short battery life is attributed to transfer of silver oxide from plate to plate and to the growth of highly branched zinc dendrites, which can cause short circuits in the battery.
The regenerated cellulosic separators in current use are incapable of withstanding degradative effects (oxidation and hydrolysis) and of preventing silver ion passage through them for a sufficiently long service time.
The oxides of silver are soluble in the potassium hydroxide electrolytes generally used in silver oxide-zinc batteries. For this reason, silver can be transferred to the zinc plate. This is undesirable because it gives rise to local cell action which brings about the self discharge of the active material on the zinc electrode. Silver oxide can also degrade separator material and form metal deposits on and in the separator, which may form metallic paths between electrodes subjecting them to internal short circuits.
When zinc is plated out of alkaline zincate solutions in the charging process, such as may occur at high current densities in silver oxide-zinc batteries, the deposit can form numerous highly branched, dendritic structures. These may enter and grow through the battery separators causing catastrophic shorting or excessive self-discharge.
To solve these problems, new separator materials must be found. Some of the desirable characteristics would include (1) the ability to prevent the transfer of silver oxides through the material, (2) a structure in which silver would not readily deposit or degrade and (3) a structure which would prevent catastrophic dendritic shorting. In addition, the battery separator must be permeable to the transport of K.sup.+ and OH.sup.- electrolyte ions, have a low electrical resistance and be electronically insulating.
Other separator materials such as methylcellulose, polyvinyl alcohol, polyethylene acrylic acid graft polymers, and chemically grafted polyethylene have not satisfied all of the above requirements, which are necessary to improve battery output.
If, in addition to ordinary service, the silver oxide-zinc battery is expected to withstand relatively high temperatures, of the order of 120.degree.-135.degree.C, such as might occur during the kind of sterilizing cycles contemplated in the case of batteries for interplanetary space flights, then the battery separator, already the weak link of the battery system, becomes even more susceptible to serious damage.
There is a need, therefore, for a separator material able to perform better than the cellulosic and other type prior art separators and, in addition, be capable of withstanding the prolonged exposure to high temperature while immersed in caustic solution.